Method of manufacturing self-tapping screws



97 c. H. ANSINGIH 3,537,288

METHOD OF MANUFACTURING SELF-TAPPING SCREWS ori inal Fild Oct. 22, 1965 I 2 Sheets-Sheet 1 INVENTOR CARL H. ANSINGH BY pal-1. M QM Nov. 3,1970 c. H. ANSINGH 3,537,283

METHOD OF MANUFACTURING SELF-TAPPING scREws Original Filed Oct. 22, 1965 2 Sheets-Sheet 2 -INVENTOR CARL H. ANSINGH BY who igmw United States Patent 3,537,288 METHOD OF MANUFACTURING SELF- TAPPING SCREWS Carl H. Ansingh, Toronto, Ontario, Canada, assignor to P. D. Robertson Mfg., Co. Limited, Milton, Ontario, Canada Original application Oct. 22, 1965, Ser. No. 500,922, now Patent No. 3,398,625. Divided and this application Aug. 5, 1968, Ser. No. 750,282

Int. Cl. B21d 17/04; B21h 3/06 US. C]. 72-88 2 Claims ABSTRACT OF THE DISCLOSURE The method of forming a self-tapping screw comprising rolling a screw blank having a head and a shank portion of constant elliptical cross section between a pair of thread rolling dies having grooves therein to form sharply crested threads, said dies being spaced apart a lesser distance adjacent the head of the blank than at the end of the blank remote from the head, the spacing being such that on rolling the elliptical blank therebetween sharply crested threads are provided only on the ends of the major axis of the elliptical cross section at the lower end of the blank while the upper end of the blank adjacent to the head is rolled into a circular uniformly threaded shank portion with the threads continuously sharply created, the maximum dimension transverse the screw axis of the lower end of the threaded shank being greater than the diameter of the circular uniformly threaded shank portion.

This application is a division of application Ser. No. 500,922 filed Oct. 22, 1965 Pat. No. 3,398,625.

BACKGROUND OF INVENTION Field of invention driving torque into a blind hole provided for example, in

structural and lower carbon content steel members, zinc or aluminum die castings, or brass or bronze forgings or the like, and which will swage form a thread formation on the wall of the untapped hole without chipping of the wall surface so that the screw can be run clear to the bottom of the hole.

Still another and important object is to provide a screw as aforesaid which will be self-locking against removal unless deliberately removed with a driver. In this connection, one important application of the invention is to provide a screw for holding two members together without requiring I the use of a nut, the screw passing through a clearance hole in the upper member and entering an untapped hole in the lower member to swage form threads therein in a manner to provide a self-locking action.

The principal feature of the invention resides in providing a screw, the lower portion or lower end of the body or shank of which is out of round, preferably elliptical, in cross section and the upper portion beneath the head is circular in cross section with the diameter of the circle being slightly smaller than the major lateral dimension of the lower portion, e.g. slightly smaller than the major axis of the lower portion when same is elliptical. With this formation the threads of the lower portion of the screw ice present deep, sharp plowing edges at opposite sides of the screw at the ends of the major lateral dimension, viz at the ends of the major axis of the elliptical cross section and relatively large relief areas formed by shallower, broader plowing edges inbetween such sharp plowing edges whereby the material of the wall surface of the hole can be progressively displaced or swaged by the lower screw portion into thread form with the relief areas providing sufficient relief so that the swaging action can take place without chipping. Since the upper circular cross sectioned portion of the screw has a slightly smaller diameter than for example, the major axis of the elliptical portion, at the ends of which the sharp plowing edges of the lower portion of the screw occur, the female threads formed in the wall surface of the hole by virtue of such plowing edges will be of slightly larger diameter than the circular male threads on the upper portion of the screw.

Thus, as a result of this thread clearance, the screw can be driven deep into the hole with a low running or driving torque. It will also be understood that any tendency of the metal of the wall surface to flow axially towards the screw head to fill in behind the advancing lower end of the screw will be such that it will not interfere with the free running of the reduced outer dimensioned upper screw portion but will provide a positive locking of the screw against accidental removal.

These and other objects and features will become apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a blank from which a screw of the present invention may be formed, the dotted point indicating that the blank may have either a blunt point or a relatively sharp point.

FIG. 2 is a perspective view of a screw embodying the invention produced from the blank of FIG. 1.

FIG. 3 is an elevational view partly broken away with the minor axis of the elliptical lower portion of the screw perpendicular to and the major axis lying in the plane of the paper.

FIG. 4 is a cross section on the line 44 taken through the circular upper portion of the screw.

FIG. 5 is a view similar to FIG. 3 with the minor axis of the elliptical lower portion of the screw parallel and the major axis perpendicular to the plane of the paper.

FIG. 6 is a vertical cross sectional view taken on the line 66 of FIG. 5.

FIG. 7 is a broken away and elevational view showing the blank of FIG. 1 about to be rolled between a pair of dies in the forming of the screw of FIG. 2.

With reference to FIGS. 1 and 7 the screws of the present invention is formed from an out of round blank 1 which is preferably of elliptical cross section by rolling between a pair of conventional flat machine thread rolling dies 2 indicated in FIG. 7, the dies preferably being inclined to the axis a-a of the screw at an angle of from about to 1 /2 whereby the dies are closer together and are adapted to squeeze the blank more adjacent to the head 3 than at the lower end of the blank 4 in the production of the screw 5 shown in FIGS. 2 to 6.

As a result of such rolling the lower end portion of the screw 5 is of elliptical cross section as best seen in FIG. 6 with the portions 6a of the threads, generally designated at 6, at the end of the major axis xx of the ellipse having a greater depth from the root to the crest than the thread portions 6b at the minor axis of the ellipse yy. Further as will be seen by comparing FIGS. 3 and 5 the thread portions 6a at the ends of the major elliptical axis x-x terminate as sharply edged plows at opposite sides of the screw (FIG. 3) While the threads 6b at the minor elliptical axis yy terminate in relatively flat crests ('FIG. 5) with the radial distance from the screw axis a-a, to the fiat crests of the thread portion 6b being less than the radial distance to the sharply edgedthread'portions 6a.

This effect is achieved in rolling by having the rolling dies 2 spaced apart sufliciently at the lower end of the blank so that there is sufficient metal at the major axis of the'blank' to flow and completely fill the thread grooves of thedie to form thesharply crested threads, whereas at theminor axis there .is not sufficientf metal to form complete sharply crested threads.

At the upper end of the screw 5 adjacent to the head 3, threads 6 are of uniform configuration and circular in cross section as will be seen from FIG. 4. This effect is achieved by having the dies closer together adjacent to the head of the screw whereby in the rolling operation suflicient metal is displaced to cause filling of the die grooves at all points around the blank so that the elliptical blank configuration is transformed into a circularly threaded screw portion with all threads terminating in sharp edges whose radial distance from the screw axis a-a is less than the radial distance out to the thread portions 6a as seen from FIG. 3 and whose radial distance from said axis a-a may be greater than the radial distance out to the thread portions 6b as shown in FIG. 5.

It will be understood that there will be a progressive change along the screw axis in moving from the elliptical lower portion of the screw depicted in FIG. 6 to the circular portion of the screw depicted in FIG. 4.

While 'the screwhas been shownas having the threads 6 running right to the head thereof these threads may terminate short of the head if desired.

Also, while the screw has been shown as having a blunt pointed portion -7 carrying the threads 6, it will be understood the'invention is applicable to screws having any shape of pointed portion such as a pyramidal conical, or blunt point with or without threads. FIG. 1 shows in dotted lines at 8 a blank having a pinch pointed end that could be used to produce a screw having an extended unthreaded point when rolled between the dies 2.

The head 3 of the screw may, of course, be provided with any suitable tool engaging socket or slot (not shown) or may be of other than round shape, for example, hexagonal, to be engaged by a suitable tool.

After rolling, the screw 5 may be hardened in any conventional manner as for instance by heat treating to any desired hardness depending upon the nature of the material into which it is intended to be inserted.

In driving a screw according to the invention into an untapped hole, it will be appreciated that the sharp plowing thread portions 6a at the lower end of the screw will act at opposite sides of the hole over a limited area to provide a swaging action to form female threads in the hole wall, with the thread portions 612 providing relief areas between the thread portions 6a. This arrangement allows for sufficient metal flow whereby the thread swaging and forming action on the hole wall can be carried out without requiring that excessive torque be applied to the screw and without giving rise to metal larger than the diameter of the circular 'cr'o'sssection'of the upper portion of the screw will therefore have a low running torque. Also, it will be appreciated that any tendency of the metal of the wall of the hole to back fill behind the advancing lower end of the screw will not be sufiicient to adversely eflect the running torque required to advance the circular portion of the screw intothe hole but'will tend to block removal of the lower end of the screw whose major axis will be greater than the diameter of the circular screw portion.

To illustrate the reduction in torque required 'to drive the screw of the invention 10-12 inch lbs. were needed as compared with a range of 17-36 inch lbs. needed with other types of screws, while the running torque was as low as 2-4 inch lbs;

While the screw illustrated hasa-single thread, it will be, appreciated it may be provided with for example, double threads and such variations may be made without departing from the spirit of the invention or :scope of the appended claims.

-I claim: x V 1. The method of forming a self-tapping screw comprising rolling a screw blank having a head and a shank portion of constant elliptical crosssection between a pair of thread rolling dies having grooves therein to form sharply crested threads, said dies being spaced apart a lesser distance adjacent the head of the blank than at the end of the blank remote from the head, the spacing being such that on rolling the elliptical blank therebe tween sharply crested threads are provided only on the ends of the major axis of the elliptical cross section at the lower end of the blank while the upper end of the blank adajcent to the head is rolled into a circular uniformly threaded shank portion with the threads continuously sharply crested, the maximum dimension transverse the screw axis of the lower end of-the threaded shank being greater than the diameter of the circular uniformly threaded shank portion.

2. The method as claimed in claim 1--in which said thread rolling dies are inclined at an angle of /2" to 1 /2 to the longitudinal axis of the screw blank.

References Cited UNITED STATES PATENTS CHARLES W. LANHAM, Primary Examiner E. M. COMBS, Assistant Examiner U.S. c1 X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 537 ,288 November 3 1970 Carl H. Ansingh It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, line 5,

"P. D. Robertson Mfg. Co. Limited," should read P. L, Robertso:

Mfg. (10.. Limited Signed and sealed this 30th day of March 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E SCHUYLER, JR. Attesting Officer Commissioner of Patents 

